The automotive industry is increasingly interested in incorporating metalized plastic parts with multiple finishes into vehicles. Utilizing multiple finishes on a single part has the advantage of potentially being less expensive than assembling multiple parts as well as having a consistent, well defined interface between finished areas. Additionally, assembling multiple parts can lead to gaps and noise from “rattle” or vibrations. These defects will not be present if a multi-part assembly can be consolidated into a single part.
The industry has incorporated multiple finishes into plastic parts by various methods. Parts molded from plateable resins can be masked and coated with a resist paint prior to plating. The partially plated part is then masked and coated with a finish paint. This method requires two separate paint processes, which adds to the cost of the product. Additionally, the interface between the plated and resisted areas is subject to “treeing” of the metal during the plating process that can lead to undesirable aesthetics at the metal/resist interface. Furthermore, the geometry of the part must be suitable for masking. Typically, a step is required at the interface for the mask to sit on in order to provide a clean interface free of overspray.
Alternatively, masked parts may be metalized in a vacuum through a physical vapor deposition (PVD) method depositing a very thin layer of metal on the non-masked areas of the part. The metalized part is then coated with an organic topcoat that is either cured by an ultraviolet (UV) or thermal process. The PVD method requires both a basecoat and a topcoat in addition to the metallization process that adds cost to the part. Additionally, the PVD process has a limited throughput compared with electroplating and sizeable capital costs for the PVD equipment. The geometry limitations outlined above would still apply to the PVD process. It has also been demonstrated that the field performance of the UV cured topcoats is not adequate to meet original equipment manufacturer (OEM) expectations. Premature coating failure of the technology in its current state is not sufficient to meet automotive exterior standards.
Sophisticated geometrically complex designs are difficult to plate because of large changes in current density over the part. For example, sharp corners, deep recesses, and narrow openings are particularly difficult to plate and usually result in very low plate thicknesses in these types of features. It is sometimes possible with longer plating times to allow enough current into the recess to cover the affected area, but this can result in excessive plate buildup in the high current areas leading to cosmetic defects, added weight, or issues with the part fitting properly on the vehicle. Additionally, the longer plating times make the part more expensive and reduce the productivity of the entire plating line. Current best practices in electroplating incorporate the use of auxiliary anodes to provide needed plate thickness to these hard to reach areas. Non-soluble anodes are specifically tooled and shaped to provide direct access to low current density areas of the part. These anodes are coated with a very expensive mixed metal oxide that has a limited lifetime. Auxiliary anodes are a very good way to provide plate thickness to low current density areas of a part, but add dramatic cost in terms of construction of the tooling, periodic recoating of the anodes, and cause breakdown of the brightener chemistries in the plating baths requiring more maintenance and control of the plating baths. This cost is so significant that commercial contracts can be lost due to this investment penalty.
Another current practice in electroplating parts with low current density areas is to modify the geometry. Deep narrow recesses that are too narrow to fit an auxiliary anode are shallowed up or the opening is widened. Sharp inside radii are increased. All of these changes modify the look of the design and can be a commercial detriment.
Finding the proper processing solution for complex geometries is critical to the overall performance of the piece. Parts on the exterior of a vehicle with insufficient plate thickness are at a risk for premature failure from corrosion. Parts with complex geometries such as wheel trim and grilles are particularly sensitive to this failure mechanism. Numerous examples exist of parts in the field that corrode down to bare plastic in low current density areas due to insufficient plate thickness which could have been avoided if auxiliary anodes had been employed in the plating process. Thus, there is an increasing need for improved methods for plating or metalizing parts.